Recently, there is an increasing interest in efforts to environmental problems such as energy saving and resource saving. In tires for automobiles, decrease in hysteresis loss and loss tangent (tan δ) of a rubber composition is known a means for fuel efficiency, and blending of silica brings about advantages such as decrease in a rolling resistance and increase in wet grip performance.
However, silica is inferior in reinforcing property and abrasion resistance as compared with carbon black. In order to improve reinforcing property and abrasion resistance, making fuel efficiency compatible with abrasion resistance by using silica having a large nitrogen absorption specific surface area or by blending two kinds of silica having different specific surface areas has been proposed (JP 2008-50570 A). However, in the case of silica having a large nitrogen absorption specific surface area, an interaction between silica particles is strong and aggregation occurs easily as compared with silica having a small nitrogen absorption specific surface area, and therefore, a Mooney viscosity of a rubber compound increases and thus, processability is lowered.
Generally in the case of blending silica to a rubber, a coupling agent is added to decrease a Mooney viscosity and increase a modulus. However, in the case of silica having a large nitrogen absorption specific surface area, since sufficient dispersion cannot be obtained and a Mooney viscosity cannot be decreased, deterioration of abrasion resistance is brought about, and thus there is a limit in dispersion of silica.
While fuel efficiency is improved by using silica and a silane coupling agent, in the case of insufficient dispersion of silica, degradation of processability, abrasion resistance and mechanical strength occurs in many cases due to increase in a Mooney viscosity. Thus, there is a room for improvement from technical point of view.